All-trans retinoic acid (ATRA), a derivative of vitamin A (retinol), is required for the appropriate cellular proliferation and differentiation of normal human prostate epithelial cells. Human prostate cancer cells contain much lower levels of ATRA than normal cells. We hypothesize that aberrant metabolism of ATRA and dysregulation of gene expression in prostate tumor cells are related to the abnormal growth properties of the tumor cells. A rationale for using retinoic acid in prostate cancer therapy is further supported by the effectiveness of ATRA in the treatment of acute promyelocytic leukemia (APL). We hypothesize that the efficacy of ATRA can be enhanced if it is administered in combination with retinoic acid metabolism blocking agents (RAMBAs) plus low doses of selective histone deacetylase inhibitors (HDACIs) such as trichostatin A(TSA) or suberoylanilide hydroxamic acid (SAHA). HDACIs will act by sensitizing prostate cancer cells towards ATRA differentiation activity. The discovery of the retinoic acid receptor (RAR)/ADAC complex provides a rationale for combining RAs and HADCIs. The goals of this proposal are to use in vitro cell culture and also mouse xenograft models to ascertain the effectiveness of ATRA, various RAMBAs plus HDACIs in inhibiting the growth and inducing the differentiation of the human prostate cell lines, LNCaP, LAPC-4, PC-3 and DU-145. A second goal of the project is to understand at the molecular level the mechanisms by which the combination treatments result in human prostate tumor cell growth inhibition. These studies may provide a much clearer rationale for new clinical treatments for prostate cancer in humans. If this approach appears successful, we will consider the need for developing novel HDAC inhibitors in the subsequent RO1 proposal on the strengths of our expertise in drug design, discovery and development. The long-term goal of this project is to develop compounds with characteristics that are likely to provide effective antitumor activity against androgen-dependent and androgen independent prostatic cancer. The following specific aims that are proposed should enable us obtain substantial data that may support our hypothesis.